Continuous polymerisation process

ABSTRACT

A process of manufacturing particles which process comprises the following steps in combination A. FEEDING INTO A CLOSED REACTOR A NUTRIENT, A RECYCLE OF PARTICLES FROM STEP (C) AND OPTIONALLY ADDING SEED MATERIAL; B. ALLOWING THE RECYCLED PARTICLES TO GROW INTO LARGER PARTICLES AND SEPARATING THE RESULTANT PARTICLES INTO SIZED FRACTIONS; AND C. RECYCLING PARTICLES OF LESS THAN A CERTAIN PREDETERMINED SIZE TO STEP (A) AND REMOVING PARTICLES LARGER THAN A CERTAIN PREDETERMINED SIZE AS PRODUCT. Apparatus wherein this process may be carried out is also described.

United States Patent 1 Busby et al. I

[11] 3,755,281 [45] Aug. 28, 1973 CONTINUOUS POLYMERISATION PROCESS [75] Inventors: Brian James Busby, Victoria; David Alexander Hughes, New South Wales, both of Australia [73] Assignee: Imperial Chemical Industries of Australia and New Zealand Limited, Melbourne, Victoria, Australia [22] Filed: May 17, 1971 [21] Appl. No.: 143,891

[30] Foreign Application Priority Data 2,979,492 4/1961 Govennale 260/928 W Primary Examiner-Joseph L. Schofer Assistant Examiner-Roger S. Benjamin Attorney-Cushman, Darby & Cushman [5 7 ABSTRACT A process of manufacturing particles which process comprises the following steps in combination a. feeding into a closed reactor a nutrient, a recycle of particles from step (c) and optionally adding seed material;

b. allowing the recycled particles to grow into larger particles and separating the resultant particles into sized fractions; and

0. recycling particles of less than a certain predetermined size to step (a) and removing particles larger than a certain predetermined size as product.

Apparatus wherein this process may be carried out is also described.

1 Claim, 1 Drawing Figure CONTINUOUS POLYMERISATION PROCESS This invention relates to a new process of manufacturing particles of a narrow predetermined size range and to new apparatus for carrying out such a process.

Usually when particles are manufactured in a liquid medium in a continuous process a mixture of particles of varying size is obtained. Although for some applications this has no deleterious effect on the value in the product, not most cases particles falling within a narrower size range are desirable. For example, when polyvinyl chloride particles the processed on a powder fed extruder higher output rates are achieved when polyvinyl chloride particles of a narrow particle size range are used than when normal polyvinyl chloride particles of wide size range are used. A further advantage is that the amount of dust formed on handling the particles is reduced if the powder does not contain the normal proportion of fine particles. A further advantage is that a consistent size distribution leads to a more consistent product and will therefore reduce the need for frequent changes in the processing conditions.

We have now found a continuous method of manufacturing particles in which the said particles may be obtained in a much narrower size range than that obtained previously.

Accordingly we provide a continuous process of manufacturing particles which process comprises the following steps in combination:

a. feeding into a closed reactor a nutrient as hereinafter defined, a recycle of particles from step (c) and optionally adding seed material as hereinafter defined;

b. allowing the recycled particles to grow into larger particles and separating the resultant particles into sized fractions: and

c. recycling particles of less than a certain predetermined size to step (a) and removing particles larger than a certain predetermined size as product.

We also provide an apparatus for manufacturing particles by a continuous process which apparatus comprises in combination, a reaction vessel, a sizing means, a recirculation means and a product removal means.

By continuous process we mean a process capable of running at a steady state with starting materials being fed in and product being withdrawn. The separation of the particles into sized fractions may be carried out in any suitable manner. Such means include, for example, the use of screens, sieves, filters, weirs or other means known for the classification of particles. However, we prefer to use centrifugal means of separating the larger particles, for example, a centrifuge or cyclone and in particular a hydrocyclone or bank of hydrocyclones. In a further embodiment the cyclone or hydrocyclone and the reactor may be combined in one vessel. By hydrocyclone we mean a cyclone designed to classify a slurry of particles in any liquid medium. The liquid medium is not restricted to water.

It is known that a hydrocyclone will normally separate particles according to their Stokes equivalent diameter, which is the diameter of a spherical particle of the same density having the same free falling velocity in a given medium. The Stokes equivalent diameter depends upon the shape and size of the particle. A hydrocyclone will not separate completely a mixture of particles into two non-overlapping size ranges. The apex product will contain substantially all the particles over a certain predetermined size as well as lesser amounts of the finer material. The vortex product will contain the remainder of the fine material. In a bank of hydrocyclones connected in series so that the apex product of one hydrocyclone is feed for the next hydrocyclone the proportion of smaller particles passing out with the coarse particles will be reduced. Thus, for example, in a bank of n hydrocyclones each designed to pass the proportion x of a certain sized particle in the apex product to the amount of that sized particle entering the hydrocyclone, the proportion of that size particle in the final apex product to the initial feed is x".

There are four important operating variables which determine the efficiency of particle separation sizes in a given hydrocyclone. These are the density, viscosity and flow rate of the liquid and the density of the particles. For given values of these variables hydrocyclones may be designed, by known means, to separate all particles into the apex product which have a Stokes equivalent diameter greater than any given predetermined size (as well as a certain proportion of the fine particles). The design of hydrocyclones is discussed in the book The Hydrocyclone by D. Bradley published by the Pergamon Press in 1965.

The proportion of fine particles in the apex product may be reduced by increasing the number of hydrocyclones used in series.

The range of particles produced by our process depends on:

a. the rate of growth of the particles;

b. the recycle rate of undersized particles; and

c. the efficiency of the separating means. For example, by using a high enough recycle rate and a sufficient number of hydrocyclones of the current design in series, any desired range of particle size may be achieved.

Accordingly we provide a continuous process of manufacturing particles which comprises the following steps in combination:

i. feeding into a closed reactor a nutrient as hereinafter defined, a recycle of small particles from step (iii) and optionally adding seed material as hereinafter defined;

ii. after allowing the seed material and recycled particles to react with the nutrient,separating the particles into two sized fractions by means of one or more hydrocyclones; and

iii. recycling the vortex product consisting of particles of less than a certain predetermined size to step (i) and removing the apex product.

By nutrient we mean throughout this specification a composition of matter which will react chemically with suitable seed material and recycled particles to form larger particles. By seed material we mean throughout this specification any small solid particles which will react chemically with a suitable nutrient and thereby grow in size. The natures of the nutrient and seed material are thus interdependent.

In some reactions the addition of seed material may be unnecessary. In these reactions the recycled particles will provide sufficient nuclei on which the reaction may occur.

Seed material may also sometimes be unnecessary in the collecting Vinyl vapour processes in which particles grow by agglomeration. In certain cases suitable seed material may be added with advantage to act as nuclei for the agglomeration of small particles. Our invention may therefore be used to produce particles of polymer by the known process of controlled latex coagulation.

In a preferred embodiment of our invention monomers may be polymerized to produce particles of polymer of reduced size range compared to the size range of particles of polymer produced in a conventional manner. In this enbodiment the nutrient is a composition comprising monomers and the seed material is any material onto which the monomers will polymerize. It will normally be necessary to add suitable known polymerization catalysts or provide other initiating means.

The character of the particles produced depends on the nature of the seed material. Choice of suitable seed material is important in the production of solid particles having desired characteristics such as suitable shape or porosity.

In our process it is essential that solid particles are producedand that substantially all the polymerization occurs as growth on existing particles or by agglomeration of existing particles. Ethylenically unsaturated monomers may be used in our process if they are polymerized under such conditions that solid particles will be produced. Thus, for example, the conditions used for the suspension polymerization of ethylenically unsaturated monomers is suitable for our process. To minimise polymerization of the monomers occurring other than by growth reactions we prefer that the conditions of reaction are such that no liquid monomer is present in the polymerization reaction mixture.

Among suitable ethylenically unsaturated monomers may be mentioned acrylamide, acrylonitrile, acrylic acid, methacrylic acid, vinyl acetate, ethylene, propylene, methylpentene-l, styrene, alkyl methacrylate, e.g., methyl methacrylate, or halo-olefins and mixtures thereof.

In particular the process may be used to prepare polyvinyl chloride and copolymers with vinyl acetate.

Our invention is most useful in the manufacture of polymer particles from monomers of limited solubility in the polymers, for example, polyvinyl chloride particles from vinyl chloride.

It is known in the prior art that in the production of polyvinyl chloride from vinyl chloride, to avoid buildup of polymer in the apparatus it is preferable to run the reaction at temperature and pressures such that the dew point of the vinyl chloride monomer is not reached and no liquid vinyl chloride can exist in the reactor. We also prefer to design that apparatus in such a manner that the parts of the apparatus exposed to the reaction medium are continuously bathed in a moving stream of reaction medium. Build-up of polymer has been found to occur on parts of the apparatus bounding trapped bubbles of vinyl chloride and also on parts of the apparatus contacting stagnant reaction medium.

The design of the apparatus and the operating conditions may be found by simple experimentation.

The invention is also of use in the production of popcorn polymers. Pop-corn polymers are polymers in which the seed material and growing particles catalyse the reaction of monomer with the particles.

The seed material may be different from the polymer formed from the nutrient. Thus heterogeneous particles may be prepared with special properties, for example, shell polymers.

Accordingly we provide a continuous process of manufacturing polymer particles which comprises the following steps in combination:

I. feeding into a closed reactor an ethylenically unsaturated monomer, a recycle of small particles from step (3), and optionally adding seed material as hereinbefore defined;

2. after allowing the recycled particles to grow separating the particles into two sized fractions by means of one or more hydrocyclones; and

3. recycling the vortex product consisting of particles of less than a certain predetermined size to step l and removing the apex product.

The hydrocyclonesmay be designed by known methods to cut the particle size range at any desired point. Thus the Stokes equivalent diameter of the product particles is preferably in the range from 10 microns to 1,000 microns, and more preferably in the range from 50 to 300 microns, for example microns.

Although the process of our invention may be carried out using a separate hydrocyclone and reaction vessel it is sometimes more convenient to use an apparatus where the hydrocyclone is enclosed in the reaction vessel. This arrangement has the advantage that the equipment enclosed does not have to withstand large pressure differences and is therefore relatively low in capital cost.

Accordingly we provide a process of producing particles of polymer which process comprises continuously supplying to an enclosed reactor,nutrient as hereinbefore defined and, optionally, seed material and continuously withdrawing from the reaction vessel particles of polymer greater than a predetermined size.

This process may be carried out in a novel apparatus and accordingly we provide a new apparatus comprising an enclosed reactor comprising an inlet in said reactor for introduction of nutrient, optionally a second inlet in said reactor for the introduction of seed material, an enclosed hydrocyclone having at its apex a means of removing the apex product and a means of applying a pressure differential across the inlet and the outlet to the hydrocyclone.

In the large scale manufacture of polymer particles according to our invention it is envisaged that the capacity of the reaction vessel and of the hydrocyclone will be matched and that the various parts of the appa ratus will all be designed to be fully utilized. For the small scale operation of our invention however, the reaction vessel need only be of sufficient size to enclose the hydrocyclone and its ancillary pump. A suitable apparatus for this purpose, for example, is shown in the accompanying drawing. This apparatus comprises an enclosed cylindrical reaction vessel 1 with dished ends, built to withstand a pressure of 250 lbs./sq.in. containing a truncated conical shell 2 with a cylindrical extension 3 at the base. The cylindrical extension 3 is closed by an annulus 4. The annulus 4 surrounds a tube 5 extending into the interior of the conical shell 2. A cylindrical shround 6 is attached to the periphery of the annulus 4. A port 7 from the apex of the conical shell is fitted with a valve and short tubular extension through the reactor vessel 1. The cylindrical extension 3 is provided with a port 8 made in such a manner that fluid pumped through the port will enter the conical shell tangentially to the sides of the conical shell. Above the annulus 4 is a centrifugal pump rotor 9 of conventional construction rotated by an axle extending through a gland 12 in the end of the reactor. The reactor vessel 1 is fitted with inlet ports 10, ll fitted with valves.

In a further embodiment of our invention, the reaction mixture is circulated by a conventional pump from the bottom of a conventional stirred reactor through a conventional hydrocyclone and back to the top of the reactor. Polymer particles having a narrow range of particle size are delivered at the apex of the hydrocyclone, the smaller polymer particles pass back to the reactor to continue growing by further polymerisation of monomer.

Alternatively, the hydrocyclone may be replaced by a number of hydrocyclones arranged so that the larger particles withdrawn slurried with water from the apex of one hydrocyclone are supplied as the feed to the succeeding hydrocyclone. The smaller particles and the remainder of the water are returned directly to the reactor.

When the hydrocyclones are each designed to cut the particle size range at the same point then it is possible to obtain particles of an extremely narrow size range from the apex of the last hydrocyclone.

The invention is now illustrated by, but in no way limited to, the following examples in which all parts are parts by weight unless otherwise specified.

EXAMPLE 1 A reactor was constructed as shown in the accompanying drawing.

The enclosed reactor 1 had an internal diameter of 8 inches and was inches long and had a capacity of 7 litres. The conical shell 2 was 5% inches long and 2% inches in diameter at the base and three-eights inch in diameter at the apex. The cylindrical extension 3 was three-fourths inch long and 2% inches in diameter. The annulus 4 was 6 inches in external diameter and onehalf inch in internal diameter. The short tube 5 projecting into the conical shell was 1% inches long and onehalf inch in diameter. The cylindrical shroud 6 was 6 inches in diameter and 5 inches long. The tangential port 8 in the cylindrical extension 3 was one-sixteenth inch from the annulus 4 and was three-eighth inch in diameter. The port 8 was cut through the walls of the cylinder 3 in such a manner as to direct any flow of fluid through the port in a tangential direction to the walls of the conical shell 2.

The reactor was fitted with a pressure gauge. A collecting vessel of 5 litres capacity was attached to the short tubular extension, from the port 7 at the apex of the conical shell.

EXAMPLE 2 The apparatus constructed in Example 1 was used to manufacture polyvinyl chloride particles.

The collecting vessel was filled completely with cold water and the valve fitted to the apex port 7 was closed.

A slurry of 300 g fine polyvinyl chloride particles (70% minimum passing through a B88 300 sieve), 1.0 g lauryl peroxide catalyst and 5000 ml of water was placed in the reaction vessel and water was then added to fill the vessel. The reaction vessel and its contents were heated to 70C and the pump rotor was turned at 300 r.p.m. The temperature was maintained at 70C and the valve to thecolle cting vessel was opened. vinyl chloride vapor at a pressure of 1 l0 120 psi was admitted through the port 10.

After 2 hours a mixture of 12 g fine polyvinyl chloride particles, 0.25 g finely ground lauryl peroxide and 12 g of water were injected through the port 11 and this addition was repeated subsequently at hourly intervals. The increase in volume of the contents of the vessel due to the addition of reactants was relieved by allowing some liquid to leak slowly through the gland 12. After 8 hours the valve fitted to the port 7 of the conical shell was closed and the collecting vessel removed. The reaction vessel was emptied and examined for signs of build-up of polymer but the internal metal surfaces of the reactor were found to be clean with no sign of build-up of polymer. It was evident that the polymerisation could have been continued for a much longer period without the need for the reaction vessel to be cleaned out.

The contents of the collecting vessel were separated from the water by filtration and 510 g of polyvinyl chloride were recovered after drying.

The polymer from this experiment had good colour, and the powder flowed well. lts heat stability was good when compounded with normal stabilisers in rigid and plasticised moulded sheets. The polymer was compared with a commercial polymer of similar type and its particle size range was narrower than that of the commercial polymer, which had a size range distribution as measured with a Coulter Counter, manufactured by Coulter Electronics Ltd., U.l(. of greater than 40 microns and 5 percent greater than microns.

EXAMPLE 3 This example demonstrates that an apparatus as illustrated in the accompanying drawing is capable of separating coarse particles from a suspension of a mixture of fine and coarse particles.

An apparatus was constructed similar to the apparatus of Example l except that the apparatus with the exception of the rotor and rotor shaft was constructed of perspex, the cylindrical shroud 6 was omitted and the annulus 4 was the same size as the base of the cone. The rotor comprised 6 sheet steel blades mounted onto a shaft at an angle of 45 to the direction of rotation, thus creating an upthrust when rotated, the rotor was 6.3 inches in diameter.

The vessel was half-filled with water and a slurry of a mixture of g of fine, and 40 g of coarse, polyvinyl chloride particles in 300 mls of water was added. The vessel was completely filled with additional water. The rotor was turned at 600 rpm and a sample of the particles from the apex of the cone was collected in a closed tube of 8 ml capacity attached to the tube 7 leading from the apex. The sample tube became filled with particles in less than 5 minutes.

The particle size distribution of the original slurry in the reaction vessel and of the slurry in the sample tube was measured using a Coulter Counter." The particle size distribution calculated on the basis of weight showed that the median size in the original slurry was 30 microns, and in the sample tube was 115 microns. The slurry in the sample tube only contained 10% by weight of particles of size less than 70 microns while 90% by weight of the particles in the original slurry were less than this size.

EXAMPLE 4 This example demonstrates that an apparatus comprising a sizing cone external to the reaction vessel is capable of separating coarse particles from a suspen- SlOl'l.

The apparatus of Example 3 was modified by placing the cone 2, annulus 4, outlet tube 7 and tube 5 exterior to the reaction vessel. An external centrifugal pump extracted slurry from the bottom of the stirred reaction vessel and injected the slurry tangentially into the cone via an adjustable choke. The slurry was returned to the reaction vessel through a pipe from the tube leading into the top of the reaction vessel 1. A horizontal circular plate 6 inches in diameter was suspended inside the reaction vessel 7 /4 inches from the top to prevent gas being sucked into the centrifugal pump.

The vessel was completely filled with slurry using the method of Example 3. The rotor inside the reaction vessel was turned at 600 rpm and the centrifugal pump was started with the choke set so that l gal/min of slurry was injected into the cone. A sample of the particles collecting at the apex of the cone was removed in the same manner as described in Example 3. The particle size distribution of the sample slurry measured by a Coulter Counter gave a median particle size of I13 microns with only 2% by weight of the particles less than 70 microns. The distribution of the original slurry was as given in Example 3.

EXAMPLE 5 This example demonstrates that the apparatus of Example 4 is capable of separating coarse particles in a continuous growth reaction.

The experiment of Example 4 was repeated except that in addition 1 g of finely ground lauryl peroxide catalyst was added to the reaction vessel. The reaction vessel was heated to 63C, the stirrer rotated at 600 rpm and vinyl chloride was injected into the reaction vessel sufficient to saturate the slurry. The centrifugal pump was started with the choke set so that 1.4 gals/- min. of slurry were injected into the cone. A sample of particles collecting at the apex of the cone was removed by the method of Example 3. The size distribution of the sample particles, measured using a Coulter Counter, showed that the median particle size was 1 l 7 microns with 3% less than 70 microns.

After the experiment the reactor, piping and cone were dismantled and examined. There was no sign of build-up observable on any of the surfaces.

EXAMPLE 6 The perspex model of Example 3 was operated with water but no polymer particles to demonstrate the behaviour of the apparatus under several operating conditrons.

The vessel was filled completely with water containing a few small pieces of paper tissue and the rotor turned at 600 rpm. The tissues spun round the sides of the vessel outside the cone starting near the lid and descending to near the bottom of the vessel in a rapid spiral. The rapid movement of tissues at the bottom of the vessel demonstrated the absence of any stagnant zone. Some of the tissues rose rapidly in a spiral near to the outside of the central cone and moved rapidly through the tangential inlet into the cone. Inside the cone the tissues spiralled very rapidly to about half way down the cone and then decreased their radius of circling to about one-fourth inch around the axis of the cone and increased their speed still further. After spinning in this fashion for a second or two they were very rapidly ejected from the cone through the vertical outlet tube 5 into the main vessel. The path followed by these pieces of tissue shows the intense spinning action inside the cone and illustrates the way in which fine particles are recirculated through the cone.

The rotor was stopped and mls of water were extracted from the vessel being replaced by air. When the rotor was spun at 600 ppm a vortex was formed round the rotor and some air was sucked into the centre of the rotor and ejected at the blade tips as small bubbles.

When a further l00 mls of water was extracted from the vessel and the rotor spun at 600 rpm the vortex was larger and a dense cloud of small bubbles was maintained at the blade tips and also deeper in the vessel. This illustrates the way monomer gas is dispersed efficiently into the reacting slurry by this particular design of rotor. A small separate air bell formed round the top sides of the cone and this grew larger when more water was extracted from the vessel and replaced by air. lnside this air bell and on the sides of the cone, build-up of polymer can form during a polymerization reaction due to splash with subsequent cementing by further polymerization. This condition was avoided by breaking up the air bell with a simple vertical plate baffle attached to the cone or alternatively by thickening the walls of the cone appropriately to fill the space occupied by the air bell, or preferably by taking the cone outside the reaction vessel anc circulating slurry to it with a pump as in Example 5.

A similar but smaller air bell which formed inside the top of the cone was removed by venting this space with an one-eighth inch hole drilled in the side of the outlet tube 5 at the top near its junction with the disc 4 covering the cone.

When more than 400 mls of the original water in the vessel was replaced by air, the vortex in the top of the vessel became so large that only the tips of the rotor blades were immersed and the spinning rate inside the cone diminished, changing the particle classification conditions and eventually preventing efficient separation of the coarser particles. The rotor was replaced by a 7% inch diameter paddle which maintained a nearly constant spinning velocity inside the cone up to a total gas space inside the reaction vessel of 600 mls and thus allowed a wider tolerance in operating conditions.

We claim:

1. In a continuous process for the catalyzed suspension polymerization of vinyl chloride in aqueous media the improvement consisting of the following steps in combination;

a. feeding vinyl chloride into a closed reactor substantially completely filled with an aqueous suspension of particles of polyvinyl chloride at such a temperature and pressure that the dew point of vinyl chloride is not reached; and such that the vinyl chloride dissolves in the polyvinyl chloride particles and polymerizes;

b. passing the aqueous suspension of particles through at least one hydrocyclone capable of separating particles larger than a certain predetermined size chosen from the range 50 to 300 microns;

c. recycling the vortex product of the hydrocyclone or hydrocyclones to step (a) and removing the apex product.

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